synch.cc

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// synch.cc 
//      Routines for synchronizing threads.  Three kinds of
//      synchronization routines are defined here: semaphores, locks 
//      and condition variables.
//
// Any implementation of a synchronization routine needs some
// primitive atomic operation.  We assume Nachos is running on
// a uniprocessor, and thus atomicity can be provided by
// turning off interrupts.  While interrupts are disabled, no
// context switch can occur, and thus the current thread is guaranteed
// to hold the CPU throughout, until interrupts are reenabled.
//
// Because some of these routines might be called with interrupts
// already disabled (Semaphore::V for one), instead of turning
// on interrupts at the end of the atomic operation, we always simply
// re-set the interrupt state back to its original value (whether
// that be disabled or enabled).
//
// Once we'e implemented one set of higher level atomic operations,
// we can implement others using that implementation.  We illustrate
// this by implementing locks and condition variables on top of 
// semaphores, instead of directly enabling and disabling interrupts.
//
// Locks are implemented using a semaphore to keep track of
// whether the lock is held or not -- a semaphore value of 0 means
// the lock is busy; a semaphore value of 1 means the lock is free.
//
// The implementation of condition variables using semaphores is
// a bit trickier, as explained below under Condition::Wait.
//
// Copyright (c) 1992-1996 The Regents of the University of California.
// All rights reserved.  See copyright.h for copyright notice and limitation 
// of liability and disclaimer of warranty provisions.

#include "copyright.h"
#include "synch.h"
#include "main.h"

//----------------------------------------------------------------------
// Semaphore::Semaphore
//      Initialize a semaphore, so that it can be used for synchronization.
//
//      "debugName" is an arbitrary name, useful for debugging.
//      "initialValue" is the initial value of the semaphore.
//----------------------------------------------------------------------

Semaphore::Semaphore(char* debugName, int initialValue)
{
    name = debugName;
    value = initialValue;
    queue = new List<Thread *>;
}

//----------------------------------------------------------------------
// Semaphore::Semaphore
//      De-allocate semaphore, when no longer needed.  Assume no one
//      is still waiting on the semaphore!
//----------------------------------------------------------------------

Semaphore::~Semaphore()
{
    delete queue;
}

//----------------------------------------------------------------------
// Semaphore::P
//      Wait until semaphore value > 0, then decrement.  Checking the
//      value and decrementing must be done atomically, so we
//      need to disable interrupts before checking the value.
//
//      Note that Thread::Sleep assumes that interrupts are disabled
//      when it is called.
//----------------------------------------------------------------------

void
Semaphore::P()
{
    Interrupt *interrupt = kernel->interrupt;
    Thread *currentThread = kernel->currentThread;
    
    // disable interrupts
    IntStatus oldLevel = interrupt->SetLevel(IntOff);   
    
    while (value == 0) {                // semaphore not available
        queue->Append(currentThread);   // so go to sleep
        currentThread->Sleep(FALSE);
    } 
    value--;                    // semaphore available, consume its value
   
    // re-enable interrupts
    (void) interrupt->SetLevel(oldLevel);       
}

//----------------------------------------------------------------------
// Semaphore::V
//      Increment semaphore value, waking up a waiter if necessary.
//      As with P(), this operation must be atomic, so we need to disable
//      interrupts.  Scheduler::ReadyToRun() assumes that interrupts
//      are disabled when it is called.
//----------------------------------------------------------------------

void
Semaphore::V()
{
    Interrupt *interrupt = kernel->interrupt;
    
    // disable interrupts
    IntStatus oldLevel = interrupt->SetLevel(IntOff);   
    
    if (!queue->IsEmpty()) {  // make thread ready.
        kernel->scheduler->ReadyToRun(queue->RemoveFront());
    }
    value++;
    
    // re-enable interrupts
    (void) interrupt->SetLevel(oldLevel);
}

//----------------------------------------------------------------------
// Semaphore::SelfTest, SelfTestHelper
//      Test the semaphore implementation, by using a semaphore
//      to control two threads ping-ponging back and forth.
//----------------------------------------------------------------------

static Semaphore *ping;
static void
SelfTestHelper (Semaphore *pong) 
{
    for (int i = 0; i < 10; i++) {
        ping->P();
        pong->V();
    }
}

void
Semaphore::SelfTest()
{
    Thread *helper = new Thread("ping");

    ASSERT(value == 0);         // otherwise test won't work!
    ping = new Semaphore("ping", 0);
    helper->Fork((VoidFunctionPtr) SelfTestHelper, this);
    for (int i = 0; i < 10; i++) {
        ping->V();
        this->P();
    }
    delete ping;
}

//----------------------------------------------------------------------
// Lock::Lock
//      Initialize a lock, so that it can be used for synchronization.
//      Initially, unlocked.
//
//      "debugName" is an arbitrary name, useful for debugging.
//----------------------------------------------------------------------

Lock::Lock(char* debugName)
{
    name = debugName;
    semaphore = new Semaphore("lock", 1);  // initially, unlocked
    lockHolder = NULL;
}

//----------------------------------------------------------------------
// Lock::~Lock
//      Deallocate a lock
//----------------------------------------------------------------------
Lock::~Lock()
{
    delete semaphore;
}

//----------------------------------------------------------------------
// Lock::Acquire
//      Atomically wait until the lock is free, then set it to busy.
//      Equivalent to Semaphore::P(), with the semaphore value of 0
//      equal to busy, and semaphore value of 1 equal to free.
//----------------------------------------------------------------------

void Lock::Acquire()
{
    semaphore->P();
    lockHolder = kernel->currentThread;
}

//----------------------------------------------------------------------
// Lock::Release
//      Atomically set lock to be free, waking up a thread waiting
//      for the lock, if any.
//      Equivalent to Semaphore::V(), with the semaphore value of 0
//      equal to busy, and semaphore value of 1 equal to free.
//
//      By convention, only the thread that acquired the lock
//      may release it.
//---------------------------------------------------------------------

void Lock::Release()
{
    ASSERT(IsHeldByCurrentThread());
    lockHolder = NULL;
    semaphore->V();
}

//----------------------------------------------------------------------
// Condition::Condition
//      Initialize a condition variable, so that it can be 
//      used for synchronization.  Initially, no one is waiting
//      on the condition.
//
//      "debugName" is an arbitrary name, useful for debugging.
//----------------------------------------------------------------------
Condition::Condition(char* debugName, Lock * conditionLock)
{
    name = debugName;
    condLock=conditionLock;
    waitQueue = new List<Semaphore *>;
}

//----------------------------------------------------------------------
// Condition::Condition
//      Deallocate the data structures implementing a condition variable.
//----------------------------------------------------------------------

Condition::~Condition()
{
    delete waitQueue;
}

//----------------------------------------------------------------------
// Condition::Wait
//      Atomically release monitor lock and go to sleep.
//      Our implementation uses semaphores to implement this, by
//      allocating a semaphore for each waiting thread.  The signaller
//      will V() this semaphore, so there is no chance the waiter
//      will miss the signal, even though the lock is released before
//      calling P().
//
//      Note: we assume Mesa-style semantics, which means that the
//      waiter must re-acquire the monitor lock when waking up.
//
//      "conditionLock" -- lock protecting the use of this condition
//----------------------------------------------------------------------

void Condition::Wait() 
{
     Semaphore *waiter;
    
     ASSERT(condLock->IsHeldByCurrentThread());

     waiter = new Semaphore("condition", 0);
     waitQueue->Append(waiter);
     condLock->Release();
     waiter->P();
     condLock->Acquire();
     delete waiter;
}

//----------------------------------------------------------------------
// Condition::Signal
//      Wake up a thread waiting on this condition, if any.
//
//      Note: we assume Mesa-style semantics, which means that the
//      signaller doesn't give up control immediately to the thread
//      being woken up (unlike Hoare-style).
//
//      Also note: we assume the caller holds the monitor lock
//      (unlike what is described in Birrell's paper).  This allows
//      us to access waitQueue without disabling interrupts.
//
//      "conditionLock" -- lock protecting the use of this condition
//----------------------------------------------------------------------

void Condition::Signal()
{
    Semaphore *waiter;
    
    ASSERT(condLock->IsHeldByCurrentThread());
    
    if (!waitQueue->IsEmpty()) {
        waiter = waitQueue->RemoveFront();
        waiter->V();
    }
}

//----------------------------------------------------------------------
// Condition::Broadcast
//      Wake up all threads waiting on this condition, if any.
//
//      "conditionLock" -- lock protecting the use of this condition
//----------------------------------------------------------------------

void Condition::Broadcast() 
{
    while (!waitQueue->IsEmpty()) {
        Signal();
    }
}

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